Effect of electromagnetic centrifugal casting on solidification microstructure of cast high speed steel roll

Using self‐made electromagnetic centrifugal casting machine, optical microscopy (OM) and D/max2200pc X‐ray diffraction, the solidification microstructure and phases of as‐cast high speed steel(HSS) roll made by sand casting, centrifugal casting and electromagnetic centrifugal casting were investigated. The experiment results show that the phases of as‐cast high speed steel (HSS) roll are alloy carbide (such as W₂C, VC, Cr₇C₃), martensite and austenite. The centrifugal casting and electromagnetic centrifugal casting can apparently improve the solidification structure of HSS roll. With the increase of electromagnetic field intensity (B), the volume fraction of austenite in the HSS solidification structure increased obviously and eutectic ledeburite decreased, the secondary carbide precipitated from the austenite is more fine and distribution of secondary carbide is more even.     

Equi-axed growth and related segregations in cast metallic alloys

Full-scale trials of DC ingots and laboratory scale directional solidification experiments have been performed to study the effect of grain structure on macro-segregation in industrial cast products. An Al alloy sheet ingot was cast with constant casting conditions (speed, superheat, cooling rate) except for the grain refiner: the first half of the ingot was non-inoculated, while the second half was inoculated. The results indicate that the extent and intensity of the centreline segregation is modified via the grain-refinement treatment: the finer the grains are, the more intense is the macro-segregation.

Numerical simulations of directional solidification of binary Al-Cu alloys have been carried out with the help of a 2D finite volume software which takes account of the movement of the liquid with respect to the solid in the mushy zone. It is possible to account for the segregation pattern of the directionally solidified ingots that exhibit columnar or coarse equi-axed grain structures. Contrarily, the intense segregation of the fine-grained ingots is not yet understood.     

Simulation of temperature field,flow field and solidification structure for Ag–28Cu–2Ge–0.4Co alloy

ABSTRACT

The temperature field, flow field and solidification structure of Ag–28Cu–2Ge–0.4Co alloy during solidification under water cooling, air cooling and slow cooling conditions were investigated, respectively. The results indicate that the temperature distribution is the most uniform and the solid–liquid phase region is the widest under slow cooling condition. The temperature gradient at the solidification front is the largest under water cooling condition. The solidification rate increases with the distance away from the sidewall under three cooling conditions. Under water-cooled conditions, the liquid flow at the front of the liquidus is much smaller than that in the other two conditions. The ratio of equiaxed crystals and columnar crystals in the solidified structure is different under different cooling conditions.     

Structural transitions in directionally solidified graphitic cast irons

Abstract

Magnesium fading in conjunction with directional solidification has been used to determine the conditions for the structural transition from flake to compacted to nodular graphite in a hypoeutectic and a hypereutectic cast iron. Results are presented in terms of predominance-area diagrams and the conditions for stability are given. Magnesium fading has also been studied during the experiments and a mass transfer coefficient has been determined.

MST/474     

Equi-axed growth and related segregations in cast metallic alloys

Full-scale trials of DC ingots and laboratory scale directional solidification experiments have been performed to study the effect of grain structure on macro-segregation in industrial cast products. An Al alloy sheet ingot was cast with constant casting conditions (speed, superheat, cooling rate) except for the grain refiner: the first half of the ingot was non-inoculated, while the second half was inoculated. The results indicate that the extent and intensity of the centreline segregation is modified via the grain-refinement treatment: the finer the grains are, the more intense is the macro-segregation.Numerical simulations of directional solidification of binary Al–Cu alloys have been carried out with the help of a 2D finite volume software which takes account of the movement of the liquid with respect to the solid in the mushy zone. It is possible to account for the segregation pattern of the directionally solidified ingots that exhibit columnar or coarse equi-axed grain structures. Contrarily, the intense segregation of the fine-grained ingots is not yet understood.     

Microstructural properties and hot pressing of rapidly solidified hypoeutectic low alloy white cast iron powders

Abstract

Hypoeutectic low alloy white cast iron powders were produced using a rapid solidification technique. The morphology and microstructural properties of these powders were investigated with respect to cooling rate and particle size. The density of hot pressed compacts as a function of parameters such as hot pressing time and pressure is described. It was found that retained austenite in the form of cells or dendrites was the main constituent of the powders. At 720°C the powder particles can be hot pressed into high density compacts that have a fine cementite–ferrite microstructure. These ultrafine grained compacts exhibited good superplasticity at elevated temperatures. An elongation to failure of 300% was observed.

MST/1682     

Revealing and characterising solidification structure of ductile cast iron

Abstract

This study deals with the solidification and microsegregation processes of near eutectic ductile cast iron. A detailed analysis of old and new solidification models was made. Metallographic techniques, developed elsewhere by the authors, were used to reveal the solidification microstructure and macrostructure. The results allow a new solidification model to be proposed, where each solidification unit is a grain of eutectic austenite that has a dendritic substructure and contains a very large number of graphite nodules. A pattern of microsegregation exists inside each solidification grain, while no intergranular microsegregation has been detected. A procedure to characterise the solidification refinement was developed considering the location of the last to freeze areas.     

Effect of microstructural variables on tensile behaviour of A356 cast aluminium alloy

Abstract

The effects of microstructural variables, including secondary dendrite arm spacing (SDAS), the size of primary α phase, the aspect ratio of eutectic Si particle and the thickness of eutectic wall structure, on tensile behaviour of A356 cast aluminium alloy, were quantitatively identified using linear regression analysis method. For systematic microstructural control of A356 specimen, directional solidification method was used with different solidification rates of 5, 25, 50 and 100 μm s^?1 respectively. The linear regression analysis suggests that each microstructural variable affects tensile strength and tensile elongation of A356 cast aluminium alloy in a similar fashion. The change in tensile behaviour with varying microstructural variables in A356 cast aluminium alloy is discussed based on fractographic and micrographic observations.     

Damping capacity of sand cast magnesium alloy AZ91

Abstract

Measurements of damping properties have been carried out on sand cast specimens of the most widely used commercial magnesium alloy AZ91. Damping properties of specimens in both as cast and heat treated (T4 and T6) conditions were determined by measurements of the logarithmic decrement of free bending beam vibration at various frequencies. The best damping properties were found in the as cast specimens. This is directly related to the microstructure, which consists of α magnesium, large inclusions of β phase (Mg₁₇Al₁₂), as well as lamellar eutectoid Mg₁₇Al₁₂ precipitating as a result of low cooling rate. For homogenised and annealed specimens, the maxima of the hardness and of the damping capacity as a function of annealing time was found. The measurements of the logarithmic decrement can be explained using a model of Granato and Lücke.     

Mechanical properties of conventionally cast,directionally solidified,and single-crystal superalloys

Abstract

In this paper the authors compare the creep and low-cycle fatigue properties of conventional, directionally solidified, and single-crystal castings produced from nickel-base superalloys. A brief historical review describes the reasons for the evolution from wrought to cast product through directionally solidified to modern single-crystal (‘monocrystal’) castings. The influence of microstructural variations produced by the casting conditions, such as porosity and grain size, on creep and low-cycle fatigue properties are illustrated. The important aspects of postsolidification heat treatment, hot isostatic pressing, and the damaging effects of impurities are described for conventional castings. The results of controlling the microstructures produced by directional solidification especially by high temperature gradient solidification are demonstrated by comparing the creep properties of directionally solidified materials with those of the conventionally cast alloys in long-term tests. The creep and low-cycle fatigue properties depend on the stress direction relative to the crystallographic directions of the material for both directionally solidified and single-crystal castings. For single crystals, individual alloys show variable dependences of properties on the crystallographic directions. Directionally solidified materials show advantages in thin sections and are less sensitive to the effects of impurities compared to conventional castings.

MST/329     

Use of titanium and zirconium in centrifugally cast heat resistant steel

Abstract

Low carbon 25Cr–35Ni steel (HP type steel) modified with titanium and zirconium has been produced by centrifugal casting. The different phases present in the as cast and aged conditions were described by light optical and scanning electron microscopy with secondary electron imaging and energy dispersive spectroscopy. Results suggest that the use of titanium as a microalloying element reduces secondary precipitation during aging. Moreover, secondary precipitates in the microalloyed steel are much finer and more evenly distributed. On the other hand, zirconium oxides was found to be potential nucleation sites for primary titanium rich carbides contributing to an optimum distribution of these carbides in the tubes. These differences together with the higher stability of the titanium containing primary carbides are responsible for the improvement on ductility and creep resistance found in the present work.     

Microstructure and properties of monotectic Cr—Cu cast irons

Abstract

A metal quasi-composite, containing copper inclusions within a chromium cast iron matrix was investigated. The dispersion of Cu particles was produced directly from the melt by cooling through the miscibility gap. The effect of casting parameters (pouring temperature and cooling rate) on the size and distribution of Cu particles has been studied using optical metallography, SEM, and TEM. It has been shown that Cu particles are responsible for improved machineability and wear resistance of Cr cast irons with 10 -12 wt-%Cu, since Cu particles break up the network of primary carbides and also act as a solid lubricant.     

Development of high strength suction cast hypereutectic Al–20Si alloy containing gamma alumina and strontium

ABSTRACT

In this research, the hypereutectic Al–20Si alloy, containing an optimised amount of 4?wt-% γ-Al₂O₃ and 0.1?wt-% strontium, was successfully synthesised through the suction casting process. The cooling rate (563?K?s^?1) involved during solidification was estimated by measuring secondary dendritic arm spacing. As a result, a significant change in the morphology of silicon was observed and also led to the formation of a small fraction of refined α-Al throughout the microstructure. The work demonstrates that high tensile properties in hypereutectic Al–20Si alloys can be achieved without the use of conventional inoculants (such as phosphorus). The suction cast Al–20Si alloy exhibits remarkably enhanced tensile properties of average ultimate tensile strength of 355?MPa and average ductility of 7.1%.     

Growth morphology and mechanism of MC carbide under quasi-rapid solidification conditions

The carbide of group IVB and group VB elements, i.e. MC carbide, is an important constitution and strengthening phase for many alloy tool steels and cast nickel-base superalloys. Since the as-solidified growth morphology, size and distribution have an important influence on both the mechanical properties and hot workability, research on the solidification behavior of MC carbide is an important subject for cast superalloys and many high alloy tool steels. The growth morphology and mechanisms of MC carbide, under slow-cooling and rapid solidification conditions, has been studied intensively as functions of the solidification cooling rate. The solidification behavior of MC carbide under quasi-rapid solidification conditions has not been reported in open literature. In this paper, the growth morphology and mechanism of an MC carbide (TiC type) under quasi-rapid solidification conditions is studied in a laser surface alloyed coating on a titanium aluminide alloy Ti–48Al–2Cr–2Nb (at.%). The growth morphology of the quasi-rapidly solidified MC carbide with a cooling rate of 4×10²°C is found to be dendritic with strong faceted, double zigzag brick-stacking growth characteristics on the dendrite arms. The growth mechanism of the MC carbide is found to be a brick-stacking/double zigzag micro-branching lateral growth from steps on the intersecting {111} planes.     

Carbide morphology in bulk and unidirectionally solidified Fe-C-V eutectics

Abstract

Investigations on the development of eutectic structures in Fe-C-V are of considerable interest as a result of the unique properties exhibited by these alloys. In this work a series of Fe-C-V alloys were prepared in order to investigate the solidification conditions and sequence, eutectic morphology, and the development of in situ eutectics. It was found that near perfect eutectic structures can be achieved by slow cooling of an Fe-C-V alloy. The eutectic obtained consisted of semispherical grains with fibrelike VC_1–x carbides. These fibres were found to develop appreciable ramifications during the solidification process. Assuming a critical area of liquid per fibre at the solidification front, as needed for stable eutectic growth, yielded a parabolic type of expression N_b ∝ t² for the number of branches (N_b) developed as afunction of the solidification time t. A qualitative model is proposed in this work to explain the different stages involved during the solidification of alloys of hypo-, hyper-, and eutectic composition. In addition, unidirectional solidification gave rise to in situ eutectics with aligned fibres. Finally, a solidification constant (u_eλ² = K) of 202 μm³ s^?1 for stable eutectic growth was experimentally determined.     

Laser Surface Melting on cast iron containing undercooled graphite

Laser surface melting experiments were carried out on an alloy cast iron with 3% C, 2% Si and 0.25% Ti containing undercooled graphite. Different heating times and energy densities were used to achieve different penetration depths and cooling rates. The microstructures obtained after surface melting and after various heat treatments are presented and discussed. It is shown how the cooling rate and concentration influence the solidification behaviour of high carbon cast irons. The composition chosen for the experiments can be considered as representative of many cast irons.     

Mechanical properties of a carbide-free bainitic cast steel with dispersed free ferrite

ABSTRACT

A high silicon, medium carbon cast steel was designed and heat-treated in order to develop microstructures composed of carbide-free bainite and small amounts of free ferrite, with the aim of obtaining high strength cast steels with improved ductility. Because of microsegregation, it was observed that ferrite present in partially austenitised samples is mostly present at the highly alloyed zones, creating an interconnected network even for low proportions of this phase. Despite the coarse solidification structure and marked microsegregation in the cast steel, the mechanical properties obtained for both fully bainitic and bainitic-ferritic microstructures largely satisfy the minimum standard requirements for high strength cast steels and are similar to those reported for wrought steels of similar microstructures.     

Modelling microstructural and mechanical properties of ferritic ductile cast iron

Abstract

It is well known that the mechanical properties of ductile cast iron (DCI) depend on its microstructure, and that the microstructure depends on the properties of the melt and the cooling conditions during casting. There have been many studies of the individual elements of the process of casting DCI, but as yet there have been very few examples of modelling the entire process to predict cooling rates, microstructure, and mechanical properties, particularly for large castings. The present paper describes a method of modelling the microstructural and mechanical properties of ferritic DCI, and applies the methods to the case of a large (13 t) thick walled (300 mm thickness) casting. The microstructure calculated includes nodule count, nodularity, ferrite grain size, and percentage ferrite. The mechanical properties calculated include yield stress, tensile strength, elongation, and static upper shelf fracture toughness (J _1C and K _JC). The calculated results compare well with those of a test casting.     

Effect of microsegregation on carbide-free bainitic transformation in a high-silicon cast steel

ABSTRACT

A high silicon cast steel was studied in the as-cast condition in order to characterise its solidification macrostructure and microsegregation. The steel, poured into 32?mm-keel-block-shaped moulds, has a coarse solidification structure and marked microsegregation, containing low-alloyed areas with a total alloy content (Cr?+?Mn?+?Si) of 2.3 wt-% and high-alloyed zones of 5.3 wt-%. The bainitic transformation behaviour at 300°C was studied at different austempering times. The bainitic reaction occurs at different rates within the specimen volume, because of its chemical heterogeneity. An austempering heat treatment leads to an inhomogeneous carbide-free bainitic microstructure with different phase amounts, morphologies and sizes. The heterogeneous distribution of sizes and chemical compositions of retained austenite is speculated to benefit mechanical properties.     

Precipitation of iron-rich intermetallic phases in Al-4.6Cu-0.5Fe-0.5Mn cast alloy

The solidification of Al-4.6Cu-0.5Fe-0.5Mn (206 type) cast alloy has been studied using Thermal Analysis, Differential Scanning Calorimeter, Scanning Electron Microscopy, Electron Back-Scattered Diffraction (EBSD), and Transmission Electron Microscopy (TEM). It is interesting to note that an iron-rich intermetallic phase, Al₃(FeMn), is experimentally observed to be dominantly present in the fully solidified cast structure of the experimental 206 cast alloy, in addition to β-Fe (Al₇Cu₂(FeMn)/Al₇Cu₂Fe) phase. Al₃(FeMn) phase is formed through a eutectic reaction approximately at 640 °C during solidification, possibly resulting from the phase selection and segregation of Fe in liquid Al. The presence of the Al₃(FeMn) phase has been confirmed by both EBSD and TEM. It is also found that both β-Fe and Al₆(FeMn) are possible to nucleate on Al₃(FeMn), as confirmed by the calculated low planar disregistries. The possible solidification reactions have been established for 206-type cast alloy at 0.5%Fe.